Three-dimensional printing

ABSTRACT

In an example of a method for three-dimensional (3D) printing, one or more dispersions is/are sprayed to form a layer including build material particles and a liquid agent. The liquid agent is evaporated from the layer to form a build material layer, and based on a 3D object model, a binder agent is applied on at least a portion of the build material layer.

BACKGROUND

Three-dimensional (3D) printing may be an additive printing process usedto make three-dimensional solid parts from a digital model. 3D printingis often used in rapid product prototyping, mold generation, mold mastergeneration, and short run manufacturing. Some 3D printing techniques areconsidered additive processes because they involve the application ofsuccessive layers of material (which, in some examples, may includebuild material, binder and/or other printing liquid(s), or combinationsthereof). This is unlike traditional machining processes, which oftenrely upon the removal of material to create the final part. Some 3Dprinting methods use chemical binders or adhesives to bind buildmaterials together. Other 3D printing methods involve at least partialcuring, thermal merging/fusing, melting, sintering, etc. of the buildmaterial, and the mechanism for material coalescence may depend upon thetype of build material used. For some materials, at least partialmelting may be accomplished using heat-assisted extrusion, and for someother materials (e.g., polymerizable materials), curing or fusing may beaccomplished using, for example, ultra-violet light or infrared light.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of examples of the present disclosure will become apparent byreference to the following detailed description and drawings, in whichlike reference numerals correspond to similar, though perhaps notidentical, components. For the sake of brevity, reference numerals orfeatures having a previously described function may or may not bedescribed in connection with other drawings in which they appear.

FIG. 1 is a graph of the Reitz diagram illustrating Reynolds number(X-axis) and Ohnesorge number (Y-axis) combinations that result indifferent jetted liquid behaviors, where the encircled area depictsdesirable numbers for the dispersions disclosed herein;

FIG. 2 is a flow diagram illustrating an example of a method for 3Dprinting;

FIG. 3 is a flow diagram illustrating an example of a method for 3Dprinting;

FIG. 4 is a flow diagram illustrating an example of a method for 3Dprinting;

FIG. 5 is a schematic illustration of a method for forming a buildmaterial layer;

FIG. 6 is a schematic illustration of a 3D printing method using abinding agent;

FIG. 7 is a schematic illustration of a 3D printing method using amasking agent;

FIG. 8 is a schematic illustration of a 3D printing method using a laserbeam;

FIG. 9A is a black and white photograph of a partially sintered layerafter it was lifted from a glass substrate;

FIG. 9B is a scanning electron microscopy (SEM) image of a cross-sectionof the partially sintered layer of FIG. 9A;

FIG. 10A is a black and white photograph of another partially sinteredlayer after it was lifted from a glass substrate;

FIG. 10B is a SEM image of a cross-section of the partially sinteredlayer of FIG. 10A; and

FIG. 11 is a SEM image of a cross-section of a multi-layered, partiallysintered structure formed via spraying and flash fusing.

DETAILED DESCRIPTION

In some methods for three-dimensional (3D) printing, build materialparticles may range in particle size and/or shape, and may be appliedusing a spreading technique. Build material including small particlesand/or irregularly shaped particles may be less expensive to producethan build material including larger, spherical particles having aGaussian particle size distribution. However, build material thatincludes medium particles (e.g., having a particle size from 20 μm toabout 30 μm) and/or small particles (e.g., having a particle size lessthan 20 μm) tends to form irregularly-shaped clumps during spreading,due to interaction (i.e., attraction) between the particles, therebyrestricting the ability of the powder to be spread into a uniform layer.Furthermore, when the dry build material includes “fines” (i.e., verysmall particles having a particle size less than 5 μm) along with themedium and/or small particles, the fines may stick to the largerparticles in the dry build material, which further restricts the abilityof the powder to be spread into a uniform layer. Additionally, dry buildmaterial that includes irregularly shaped particles is very difficult tospread into a uniform layer. Non-uniform layers can lead to 3D partshaving imprecise shapes, varying structural properties, structuraldefects and/or varying visual qualities.

In examples of the method for 3D printing disclosed herein, one or moredispersions is/are sprayed to form a layer including build materialparticles and a liquid agent, and the liquid agent is evaporated fromthe layer. This forms a build material layer. Forming the build materiallayer in this manner enables a thin layer (e.g., less than 50 μm thickdown to about 5 μm thick) of build material particles to be formed thatalso has a substantially uniform thickness. As used herein,“substantially uniform thickness” may refer to a layer that has a height(i.e., in the Z direction) with less than 30% variation along its entirelength (i.e., in the X direction) and along its entire width (i.e., inthe Y direction). In some instances, the variation in height along theentire length and width is less than 25%, or less than 20%. These thinand substantially uniform layers can be obtained even when smallparticles and/or irregularly shaped particles are used.

The spraying method of forming build material layers may be suitable foruse in a variety of three-dimensional printing methods.

Some examples of 3D printing disclosed herein may utilize a binder agentto pattern layers of the build material to form a green part (referredto herein as “3D intermediate part”), which is subsequently sintered toform a 3D object. The binder agent may include a polymer binder, such aspolymer latex particles, that when cured, temporarily hold together thebuild material particles of the 3D intermediate part. The 3Dintermediate part may be moved from the build area platform 26 to aseparate device for heating to remove the binder particles and to sinterthe build material particles of the 3D intermediate part to form the 3Dobject.

Other examples of 3D printing disclosed herein may utilize a maskingagent (e.g., a positive masking agent or a negative masking agent) topattern the layer of build material. In these examples, an entire buildmaterial layer is exposed to high energy radiation causing heating ofthe particles. When a positive masking agent is used, the patternedregion (which, in some instances, is less than the entire layer) of thebuild material layer is sintered or fused to become a layer of a 3Dobject. When a negative masking agent is used, the non-patterned regionof the build material layer is sintered or fused to become a layer of a3D object.

Still other examples of 3D printing disclosed herein may utilizeselective laser sintering (SLS) or selective laser melting (SLM). Duringselective laser sintering or melting, a laser beam is aimed at aselected region (which, in some instances, is less than the entirelayer) of a layer of build material. Heat from the laser beam causes thebuild material under the laser beam to sinter or fuse.

Throughout this disclosure, a weight percentage that is referred to as“wt % active” and a volume percentage that is referred to as “vol %active” refers to the loading (respectively based on weight or volume)of an active component of a dispersion or other formulation that ispresent in the dispersion(s), the binder agent, and/or the maskingagent. For example, polymer binder particles may be present in awater-based formulation (e.g., a stock solution or dispersion) beforebeing incorporated into the binder agent. In this example, the wt %actives of the polymer binder particles accounts for the loading (as aweight percent) of the polymer binder particle solids that are presentin the binder agent, and does not account for the weight of the othercomponents (e.g., water, etc.) that are present in the stock solution ordispersion with the polymer binder particles. Similarly, the vol %actives of the polymer binder particles accounts for the loading (asvolume percent) of the polymer binder particle solids that are presentin the binder agent, and does not account for the volume of the othercomponents (e.g. water, etc.) that are present in the stock solution ordispersion with the polymer binder particles. The term “wt % (or vol.%),” without the term actives, refers to either i) the loading (in thedispersion(s), the binder agent, and/or the masking agent) of a 100%active component that does not include other non-active componentstherein, or ii) the loading (in the dispersion(s), the binder agent,and/or the masking agent) of a material or component that is used “asis” and thus the wt % (or vol. %) accounts for both active andnon-active components.

Dispersions

Examples of the dispersion(s) disclosed herein include build materialparticles and a liquid agent. In some examples, the dispersion(s)consist(s) of the build material particles and the liquid agent with noother components. In other examples, the dispersion(s) may include othercomponents, such as a dispersant and/or an additive. In yet otherexamples, the dispersion(s) consist(s) of the build material particles,the liquid agent, and the dispersant with no other components. In stillother examples, the dispersion(s) consist(s) of the build materialparticles, the liquid agent, the dispersant, and the additive, with noother components.

In examples of the methods disclosed herein, one or more dispersions maybe used. When one dispersion is sprayed to form a layer, the componentsof the sprayed layer (e.g., the build material particles, the liquidagent, etc.), prior to evaporation, are the components of thedispersion. When multiple dispersions are sprayed to form a layer, thecomponents of the sprayed layer, prior to evaporation, include thecomponents of each of the multiple dispersions. In these examples, thecomponents of a single layer may be included in separate dispersionsprior to the formation of the layer. In these examples, each dispersionmay include at least some of the liquid agent and at least some of thebuild material particles that end up in the sprayed layer. In oneparticular example, one of the one or more dispersions includes at leastsome of the liquid agent (that is present in the sprayed layer), atleast some of the build material particles (that are present in thesprayed layer), and a dispersant (that is present in the sprayed layer).

As used herein, “dispersion” or “dispersion(s)” may refer to: (i) one ofthe one or more dispersions, (ii) at least one of the one or moredispersions, or (iii) each of the one or more dispersions.

The dispersion(s) disclosed herein are sprayable. As used herein,“sprayable” refers to the ability of the dispersion(s) to be ejectedfrom a nozzle in a jet or stream and to break into droplets (whichcontain build material particles) immediately after ejection. Thephysical properties of the dispersion (e.g., density, surface tension,and viscosity) enable the jet or stream to break into droplets.

The density is highly dependent upon the build material that is used andthe surface tension is equal to the surface tension of the liquid inwhich the build material particles are dispersed. As such, theseparameters may vary depending upon the dispersion components.

In an example, the dynamic viscosity may be determined using Einstein'sformula:

μ=μ_(l)(1+2.5φ)

where μ_(l) is the dynamic viscosity of liquid phase of the dispersionand φ is the volume concentration of solid particles in the dispersion.The dynamic viscosity varies exponentially with temperature (e.g., thetemperature of the printing environment), according to the Reynold'smodel:

μ(T)=μ₀ exp(−bT)

where T is the temperature and μ₀ and b are constants. As an example, atleast one of the one or more dispersions has a viscosity ranging fromabout 0.1 mPa·sec (0.0001 Pa·sec) to about 50 mPa·sec (0.5 Pa·sec) at20° C. Table 1 illustrates some example dynamic viscosities fordispersions with different loadings of stainless steel particles (SSP)in different liquid agents (water or isopropyl alcohol) at differenttemperatures.

TABLE 1 Temperature Dynamic Viscosity Dispersion (° C.) (Pa * s) H₂O; 50vol % SSP 20 2.48 * 10⁻³ 80  7.1 * 10⁻⁴ H₂O; 20 vol % SSP 20 1.65 * 10⁻³80  4.4 * 10⁻⁴ IPA; 50 vol % SSP 20 4.95 * 10⁻³ 80 2.07 * 10⁻³ IPA; 20vol % SSP 20  3.3 * 10⁻³ 80 1.23 * 10⁻³

To obtain a substantially uniform layer of the dispersion disclosedherein, it is desirable for the jetted stream to be within the encircledarea of FIG. 1 (within a second wind-induced regime or an atomizationregime on the Reitz diagram) so that desirably small droplets areformed. In the first (Rayleigh) regime, the jet velocity is relativelylow, resulting in laminar flow through the nozzle. This jetteddispersion is broken into large droplets away from the nozzle. In thesecond (1^(st) wind-induced) regime, the jet velocity is increased andturbulence is initiated, which causes oscillations on the jet or stream.This jetting dispersion is more broken up, resulting in smaller dropletsthan those observed in the first regime. The effect observed in thesecond regime is enhanced in the third (2^(nd) wind-induced) regime,leading to even smaller droplets which are desirable in the examplesdisclosed herein. In the fourth (atomization regime), the jetdisintegrates within the nozzle and the droplets are even smaller. Inthe examples disclosed herein, any dispersion having values in thefourth regime or near the border of the third and fourth regimes (asshown in the encircled area in FIG. 1 ) may be used.

The Reitz diagram in FIG. 1 may be used to identify suitable Ohnesorgeand Reynolds numbers for the dispersions so that they are capable ofjetting within one of the two regimes, and the following calculationsmay be used to identify suitable physical properties (e.g., density,dynamic viscosity, surface tension) based on the Ohnesorge and Reynoldsnumbers.

i) Reynolds number R_(e) (elucidating mixture destabilizing forces):

$R_{e} = \frac{{vL}\rho}{\mu}$

where v is the ejected jet velocity (see ii below), L is the diameter ofthe nozzle used, ρ is the density of the dispersion, and μ is thedynamic viscosity;ii) Bernoulli's formula (for ejected jet velocity):

$v = \sqrt{\frac{2\Delta p}{\rho}}$

Δp is the pressure differential between the dispersion inside the sprayvessel and the surrounding environment into which the pressure issprayed (surrounding environment=atmospheric pressure) and ρ is thedensity of the dispersion;iii) Ohnesorge number O_(h):

${O_{h} = \frac{\sqrt{W_{e}}}{R_{e}}},$

where R_(e) is Reynolds number and W_(e) is the Weber number (see ivbelow);iv) Weber number (elucidating mixture stabilizing forces):

$W_{e} = \frac{v^{2}L\rho}{\sigma}$

where v is the ejected jet velocity, L is the diameter of the nozzleused, ρ is the density of the dispersion, and σ is the surface tensionof sprayed mixture.

Each of the components of the dispersion(s) will now be described inmore detail.

Build Material Particles

The build material particles may include polymer particles, ceramicparticles, and/or metal particles. The composition of the build materialparticles may depend, at least in part, on the 3D printing process usedand/or the 3D object to be formed.

In some examples of the methods disclosed herein, the build materialparticles are selected from the group consisting of polymer particles,ceramic particles, metal particles, and combinations thereof. In otherexamples, the build material particles are selected from the groupconsisting of ceramic particles, metal particles, and combinationsthereof.

In some examples, the build material particles consist of a single typeof particles. In one of these examples, the build material particlesconsist of polymer particles with no other particles. In another ofthese examples, the build material particles consist of ceramicparticles with no other particles. In still another of these examples,the build material particles consist of metal particles with no otherparticles.

In other examples, the build material particles may include a mixture ofparticles. In some examples of the methods disclosed herein, the buildmaterial particles include a mixture of two or more metals or a mixtureof a metal and a ceramic. In other examples, the build materialparticles may include a mixture of two or more polymers, a mixture oftwo or more ceramics, or a mixture of a polymer and a ceramic. In stillother examples, the build material particles may include a mixture of apolymer and a metal.

In some examples, the build material particles include polymerparticles. In these examples, the polymer particles may include anypolymeric material suitable for 3D printing.

In some of these examples, the polymer particles may include acrystalline or semi-crystalline polymer. Some examples ofsemi-crystalline polymers include polyamides (PAs), such as polyamide 11(PA 11/nylon 11), polyamide 12 (PA 12/nylon 12), polyamide 6 (PA 6/nylon6), polyamide 4,6 (PA 4,6/nylon 4,6), polyamide 13 (PA 13/nylon 13),polyamide 6,13 (PA 6,13/nylon 6,13), polyamide 8 (PA 8/nylon 8),polyamide 9 (PA 9/nylon 9), polyamide 66 (PA 66/nylon 66), polyamide 612(PA 612/nylon 612), polyamide 812 (PA 812/nylon 812), polyamide (PA912/nylon 912), etc. Other examples of crystalline or semi-crystallinepolymers suitable for use as the polymer particles include polyethylene,polypropylene, and polyoxomethylene (i.e., polyacetals). Still otherexamples of suitable polymer particles include polystyrene,polycarbonate, polyester, polyurethanes, other engineering plastics, andblends of any two or more of the polymers listed herein. One example ofa suitable polyester is polybutylene terephthalate (PBT).

In some examples, the polymer particles may include a thermoplasticelastomer. Some examples of thermoplastic elastomers include athermoplastic polyamide (TPA), a thermoplastic polyurethane (TPU), astyrenic block copolymer (TPS), a thermoplastic polyolefin elastomer(TPO), a thermoplastic vulcanizate (TPV), and a thermoplasticcopolyester (TPC). In an example, the thermoplastic elastomer is athermoplastic polyamide. Thermoplastic polyamide elastomers arethermoplastic elastomer block copolymers based on nylon and polyethersor polyesters. Examples of TPA elastomers include polyether block amideelastomers. Polyether block amide elastomers may be obtained by thepolycondensation of a carboxylic acid terminated polyamide (PA 6, PA 11,PA 12) with an alcohol terminated polyether (e.g., polytetramethyleneglycol (PTMG), polyethylene glycol (PEG), etc.). Two examples ofcommercially available PEBA elastomers include those known under thetradename of PEBAX® (Arkema) and VESTAMID® E (Evonik Industries). Inanother example, the thermoplastic elastomer is a thermoplasticpolyurethane. Thermoplastic polyurethane elastomers may be obtained byreaction of: (i) diisocyanates with short-chain diols (so-called chainextenders) and/or (ii) diisocyanates with long-chain diols. Two examplesof commercially available TPU elastomers include those known under thetradename of DESMOPAN® (Covestro) and ELASTOLLAN® (BASF Corp.).

When the polymer particles include a crystalline or semi-crystallinepolymer, the crystalline or semi-crystalline polymer may have a wideprocessing window of greater than 5° C., which can be defined by thetemperature range between the melting point and the re-crystallizationtemperature. In an example, the polymer particles may have a meltingpoint ranging from about 50° C. to about 300° C. As other examples, thepolymer particles may have a melting point ranging from about 155° C. toabout 225° C., from about 155° C. to about 215° C., about 160° C. toabout 200° C., from about 170° C. to about 190° C., or from about 182°C. to about 189° C. As still another example, the polymer particles maybe polyamide particles having a melting point of about 180° C.

When the polymer particles include a thermoplastic elastomer, thethermoplastic elastomer may have a melting range within the range offrom about 130° C. to about 250° C. In some examples (e.g., when thethermoplastic elastomer is a polyether block amide), the thermoplasticelastomer may have a melting range of from about 130° C. to about 175°C. In some other examples (e.g., when the thermoplastic elastomer is athermoplastic polyurethane), the thermoplastic elastomer may have amelting range of from about 130° C. to about 180° C. or a melting rangeof from about 175° C. to about 210° C.

The polymer particles disclosed herein may absorb some of the radiationthat is used in 3D printing. In some instances, the absorptivity of thepolymer particles at a particular wavelength is 75% or more (e.g., 80%,90%, 95%, etc.) As examples, the polymer particles substantially absorbradiation having a wavelength within the range of 400 nm to 1400 nm. Insome examples, the absorption may be enhanced, e.g., by using a positivemasking agent, so that build material patterned with the positivemasking agent fuses, but non-patterned build material does not fuse. Inother examples, the absorption may be blocked, e.g., by using a negativemasking agent, so that so that non-patterned build material fuses, butbuild material patterned with the negative masking agent does not fuse.

In some examples, the build material particles include ceramicparticles. In these examples, the ceramic particles may include anyceramic material suitable for 3D printing.

In some of these examples, the ceramic particles may be selected fromthe group consisting of metal oxides, inorganic glasses, carbides,nitrides, borides, and combinations thereof. Some specific examples ofsuitable ceramic particles include alumina (Al₂O₃), glass, Na₂O/CaO/SiO₂glass (soda-lime glass), borosilicate glass, alumina silica glass,silicon mononitride (SiN), silicon dioxide (SiO₂), zirconia (ZrO₂),titanium dioxide (TiO₂), MgAl₂O₄, tin oxide, yttrium oxide, hafniumoxide, tantalum oxide, scandium oxide, niobium oxide, vanadium oxide,and combinations thereof. As an example of one suitable combination, 30wt % glass may be mixed with 70 wt % alumina.

The ceramic particles may have a melting point ranging from about 1000°C. to about 4000° C. As an example, the ceramic particles may be metaloxide particles having a melting point ranging from about 1000° C. toabout 2800° C.

In some examples, the build material particles include metal particles.In these examples, the metal particles may include any metal materialsuitable for 3D printing.

In one of these examples, the metal particles may be a single phasemetallic material composed of one element. In this example, sinteringmay occur below the melting point of the single element. In anotherexample, the metal particles are composed of two or more elements, whichmay be in the form of a single phase metallic alloy or a multiple phasemetallic alloy. In these other examples, sintering may occur over arange of temperatures. With respect to alloys, materials with a metalalloyed to a non-metal (such as a metal-metalloid alloy) can be used aswell.

In some examples, the metal particles may be selected from the groupconsisting of aluminum, aluminum alloys, titanium, titanium alloys,copper, copper alloys, cobalt, cobalt alloys, chromium, chromium alloys,nickel, nickel alloys, vanadium, vanadium alloys, tin, tin alloys,tungsten, tungsten alloys, tungsten carbide, tantalum, tantalum alloys,molybdenum, molybdenum alloys, magnesium, magnesium alloys, gold, goldalloys, silver, silver alloys, zirconium, zirconium alloys, ferrousalloys, stainless steel, steel, and an admixture thereof. Specific alloyexamples can include AlSi 10 Mg, 2xxx series aluminum, 4xxx seriesaluminum, CoCr MP1, CoCr SP2, Maraging steel MS1, HASTELLOY™ C,HASTELLOY™ X, NickelAlloy HX, INCONEL™ IN625, INCONEL™ IN718, stainlesssteel GP1, stainless steel 17-4PH, stainless steel 316L, stainless steel430L titanium 6Al4V, and titanium 6Al-4V ELl7. While several examplealloys have been provided, it is to be understood that other alloys maybe used.

The temperature(s) at which the metal particles sinter together mayrange from about 500° C. to about 3,500° C. In some examples, the metalparticles may have a melting point ranging from about 500° C. to about3,500° C. In other examples, the metal particles may be an alloy havinga range of melting points.

Other examples of suitable build materials may include carbon nanotubesor graphene platelets.

The build material particles may be similarly sized or differentlysized. In some examples of the methods disclosed herein, the buildmaterial particles have an average particle size ranging from about 0.1μm to about 100 μm. The term “average particle size”, as used herein,may refer to a number-weighted mean diameter or a volume-weighted meandiameter of a particle distribution. In some examples, the buildmaterial particles may have a D50 particle size distribution value thatmay range from about 1 μm to about 100 μm, from about 1 μm to about 50μm, etc. Individual particle sizes may be outside of these ranges, asthe “D50 particle size” is defined as the particle size at which abouthalf of the particles are larger than the D50 particle size and abouthalf of the other particles are smaller than the D50 particle size (byweight based of the build material particles).

The shape of the build material particles may be spherical,non-spherical, irregularly shaped (e.g., platelet-shaped), randomshapes, or a combination thereof. In some examples of the methodsdisclosed herein, the build material particles may be irregularlyshaped. In an example, irregularly shaped build material particles mayhave an aspect ratio ranging from about 2 to about 100. As one example,when the build material particles include polymer particles, the polymerparticles may be formed of, or may include, short fibers (having alength that is greater than their width) that may, for example, havebeen cut into short lengths from long strands or threads of material. Asanother example, when the build material particles include metalparticles, the metal particles may be formed by grinding, which mayproduce irregularly shaped metal particles. As still another example,when the build material particles include metal particles, the metalparticles may be formed by atomization.

In some examples of the methods disclosed herein, the build materialparticles are present in the dispersion in an amount ranging from about5 vol % to about 60 vol %, based on a total volume of the dispersion. Inan example of the methods disclosed herein, the build material particlesmay be present in the dispersion in an amount ranging from about 30 vol% to about 60 vol %, based on a total volume of the dispersion. Inanother example of the methods disclosed herein, one of the one or moredispersions includes at least some of the liquid agent and at least someof the build material particles, and the at least some of the buildmaterial particles are present in an amount ranging from about 5 vol %to about 60 vol %, based on a total volume of the one of the one or moredispersions.

Liquid Agents

The dispersion(s) disclosed herein also include the liquid agent. Theliquid agent may be selected to be compatible with the build materialparticles that are to be used in the dispersion. As used herein,“compatible with the build material particles” means that the buildmaterial particles used do not react with or dissolve in the liquidagent. The liquid agent may also be selected so that the build materialparticles are able to be dispersed therein, and so that thedispersion(s) is/are sprayable. Further, the liquid agent may evaporateat an ambient printing temperature or at a heating temperature. As usedtherein, the phrase “ambient printing temperature” may refer to thetemperature of the environment in which the 3D printing process isperformed (e.g., a temperature ranging from about 40° C. to about 50°C.). The heating temperature may be any temperature above the ambientprinting temperature that does not fuse or sinter the build material. Inan example, the heating temperature may be the temperature at which thespread build material particles are held and/or to which the spreadbuild material particles are heated after spraying.

The liquid agent used in the dispersion(s) may depend, at least in part,on the build material particles used, whether other components, such asa dispersant, are included in the dispersion(s), and/or the ambientprinting temperature. In some examples of the methods disclosed herein,the liquid agent is selected from the group consisting of water,n-propanol, isopropanol, methanol, pentane, n-hexane, ethanol, aceticacid, n-butanol, ethyl acetate, butyl alcohol, ether, and misciblecombinations thereof. As mentioned herein, the liquid agent is selectedso that it does not dissolve the build material particles, and thus isdependent upon the build materials being used.

In some examples of the methods disclosed herein, the liquid agent ispresent in the dispersion in an amount ranging from about 40 vol % toabout 95 vol %, based on a total volume of the dispersion. In an exampleof the methods disclosed herein, the liquid agent may be present in thedispersion in an amount ranging from about 40 vol % to about 70 vol %,based on a total volume of the dispersion. In another example of themethods disclosed herein, one of the one or more dispersions includes atleast some of the liquid agent and at least some of the build materialparticles (that are present in the sprayed layer), and the at least someof the liquid agent is present in an amount ranging from about 40 vol %to about 95 vol %, based on a total volume of the one of the one or moredispersions.

Dispersants

The dispersion(s) disclosed herein may include a dispersant to helpdisperse the build material particles in the liquid agent. Thedispersant may also be removable during the 3D printing process orcompatible with the 3D object formed. As used herein, “compatible withthe 3D object formed” means that the dispersant does not deleteriouslyeffect mechanical and/or aesthetic properties of the 3D object formed.

The dispersant used in the dispersion(s) may depend, at least in part,on the build material particles used, the liquid agent used, and/or the3D printing process used. Examples of the dispersant include anionicdispersants, nonionic dispersants, and cationic dispersants.

Suitable anionic dispersants include carboxylates, sulfonates, petroleumsulfonates, alkylbenzenesulfonates, naphthalenesulfonates, olefinsulfonates, alkyl sulfates, sulfates, sulfated natural oils and fats,sulfated esters, sulfated alcanolamides, and alkylphenols. One specificexample of a suitable anionic dispersant is dioctyl sodiumsulfosuccinate (also known as Aerosol OT).

Suitable nonionic dispersants include ethoxylated aliphatic alcohols,polyoxyethylene surfactants, carboxylic esters, polyethylene glycolesters, anhydrosorbitol esters and their ethoxylated derivatives, glycolesters and fatty acids, carboxylic amides, monoalkanolamine condensates,and polyoxyethylene fatty acid amides. Specific examples of suitablenonionic dispersants include branched octylphenoxypoly(ethyleneoxy)ethanol (commercially available as IGEPAL® CA-630 fromRhodia Operations), polyoxyethylene octyl phenyl ether (also known asTRITON™ X-100 from The Dow Chemical Co.), polysorbate 20 (also known asTWEEN™ 20), polysorbate 80 (also known as TWEEN™ 80), sorbitan laurate(commercially available as SPAN® 20 from Uniqema Americas), sorbitanoleate (commercially available as SPAN® 80 from Uniqema Americas). Insome examples, submicron polymer particles may be used as the nonionicdispersant.

Suitable cationic dispersants include quaternary ammonium salts, amineswith amide linkage, polyoxyethylene alkyl and alicyclic amines, N,N,N′N′trakis substituted ethylenediamines, and 2-alkyl 1-hydroxethyl2-imidazolines.

Amphoteric dispersants may also be used. Suitable amphoteric dispersantsinclude amphoteric surfactants containing both an acidic and a basichydrophilic moiety in their surface. One specific example of a suitableamphoteric dispersant is N-coco 3-aminopropionic acid and sodium salt.

In some examples, a single dispersant is used in the dispersion(s). Inother examples, a combination of dispersants is used in thedispersion(s).

Whether a single dispersant is used or a combination of dispersants isused, the total amount of dispersant(s) in the dispersion(s) may rangefrom about 0.001 vol % active to about 0.1 vol % active based on thetotal volume of the dispersion(s).

Additives

The dispersion(s) disclosed herein may also include an additive. In someexamples, the additive is selected from the group consisting of anantioxidant, a humectant, a surfactant, an antimicrobial agent, aviscosity modifier, a pH adjuster, a chelating agent, an adhesionpromoter, an anti-foaming agent, a deodorant, and a combination thereof.

Antioxidant(s) may be added to the dispersion(s) when the build materialparticles include polymer particles. The antioxidant(s) may prevent orslow molecular weight decreases of the polymer particles and/or mayprevent or slow discoloration (e.g., yellowing) of the polymer particlesby preventing or slowing oxidation of the polymer particles. In someexamples, the antioxidant may discolor upon reacting with oxygen, andthis discoloration may contribute to the discoloration of the buildmaterial. The antioxidant may be selected to minimize thisdiscoloration. In some examples, the antioxidant may be a radicalscavenger. In these examples, the antioxidant may include IRGANOX® 1098(benzenepropanamide,N,N′-1,6-hexanediylbis(3,5-bis(1,1-dimethylethyl)-4-hydroxy)), IRGANOX®254 (a mixture of 40% triethylene glycolbis(3-tert-butyl-4-hydroxy-5-methylphenyl), polyvinyl alcohol anddeionized water), and/or other sterically hindered phenols. In otherexamples, the antioxidant may include a phosphite and/or an organicsulfide (e.g., a thioester). The antioxidant may be in the form of fineparticles (e.g., having an average particle size of 5 μm or less) thatare mixed into the dispersion(s). In an example, the antioxidant may beincluded in the dispersion(s) in an amount ranging from about 0.01 wt %to about 5 wt %, based on the weight of the polymer particles. In otherexamples, the antioxidant may be included in the dispersion(s) in anamount ranging from about 0.01 wt % to about 2 wt % or from about 0.2 wt% to about 1 wt %, based on the weight of the polymer particles.

The dispersion(s) may also include a humectant, a surfactant, anantimicrobial agent, a viscosity modifier, a pH adjuster, and/or achelating agent, each of which is described below in reference to theliquid vehicle of the binder agent. As such, the dispersion(s) mayinclude any of the components described below in reference to the liquidvehicle of the binder agent in any of the amount described below (withthe amount(s) being based on the total weight of the dispersion(s)rather than the total weight of the binder agent).

Other suitable additives include an adhesion promoter to help thesprayed dispersion stick to an underlying layer of build materialparticles, an anti-foaming agent to reduce or prevent foaming of theprinted dispersion, and/or a deodorant to impart a desired odor to the3D part being formed. Any suitable adhesion promoter, anti-foaming agentand/or deodorant may be used, alone or in combination with any one ormore of the other additives.

Printing Methods and Methods of Use

Referring now to FIGS. 2 through 4 , examples of methods 100, 200, 300are depicted. The examples of the methods 100, 200, 300 may use examplesof the dispersion disclosed herein to form 3D objects.

As shown in FIG. 2 , the method 100 for three-dimensional (3D) printingcomprises: spraying one or more dispersions to form a layer includingbuild material particles and a liquid agent (reference numeral 102);evaporating the liquid agent from the layer to form a build materiallayer (reference numeral 104); and based on a 3D object model,selectively applying a binder agent on at least a portion of the buildmaterial layer (reference numeral 106). Examples of the method 100 willbe further described below in the section labeled “Printing with BinderAgents” and in reference to FIG. 6 .

As shown in FIG. 3 , the method 200 for three-dimensional (3D) printingcomprises: spraying one or more dispersions to form a layer includingbuild material particles and a liquid agent (reference numeral 202);evaporating the liquid agent from the layer to form a build materiallayer (reference numeral 204); based on a 3D object model, selectivelyapplying a masking agent on at least a portion of the build materiallayer (reference numeral 206); and exposing the build material layer toradiated energy (reference numeral 208). Examples of the method 200 willbe further described below in the section labeled “Printing with MaskingAgents” and in FIG. 7 .

As shown in FIG. 4 , the method 300 for three-dimensional (3D) printingcomprises: spraying one or more dispersions to form a layer includingbuild material particles and a liquid agent (reference numeral 302);evaporating the liquid agent from the layer to form a build materiallayer (reference numeral 304); and based on a 3D object model,selectively exposing the at least a portion of the build material layerto a laser (reference numeral 306). Examples of the method 300 will befurther described below in the section labeled “Printing using SLS/SLM”and in FIG. 8 .

While not shown, any example of the methods 100, 200, 300 may includeforming the dispersion(s). In an example, the dispersion(s) is/areformed prior to spraying the dispersion(s) to form the layer. Eachdispersion may be formed by mixing its components (i.e., at least someof the liquid agent, at least some of the build material particles,etc.) together. The dispersion may be formed in advance of spraying(e.g., suspension mixed and loaded into a spaying assembly) or theindividual components may be loaded into a sprayer and mixed just beforespraying.

Furthermore, prior to execution of any examples of the methods 100, 200,300, it is to be understood that a controller may access data stored ina data store pertaining to a 3D part/object that is to be printed. Forexample, the controller may determine the number of build materiallayers that are to be formed, the locations at which any of the agentsis/are to be deposited on each of the respective layers, etc.

Forming Build Material Layers

As mentioned above, in examples of the methods 100, 200, 300 disclosedherein, forming a build material layer includes spraying one or more ofthe dispersions. The formation of a single build material layer 10 isshown in FIG. 5 . In some examples (as shown in FIG. 5 ), a singledispersion 12 is sprayed. In other examples, multiple dispersions aresprayed.

As used herein, “spraying” refers to the ejection of a jet or stream 14of the dispersion 12 wherein the jet or stream 14 breaks into droplets16 immediately after ejection. The ejection of the jet or stream 14 maybe accomplished using a pressure gradient (i.e., pressure differentialbetween the dispersion inside the spray vessel/chamber and thesurrounding environment into which the dispersion is sprayed). Thepressure gradient forces the dispersion 12 through a nozzle 18 to formthe jet or stream 14. In some examples, the pressure gradient may rangefrom about 1 atm to about 100 atm, or from about 1.5 atm to about 50atm, or from about 2 atm to about 10 atm. After ejection, the jet orstream 14 breaks into droplets 16. Depending upon the jet velocity, thejet or stream 14 can be broken into smaller droplets, or can bedisintegrated (atomized) to form even smaller droplets. The droplets 16may have a diameter ranging from about 10 times to about 500 timessmaller than the diameter of the nozzle 18 through which the jet orstream 14 was ejected. The spray apparatus and spray parameters (i.e.,the properties of the dispersion 12, the liquid agent 24, and the buildmaterial particles 22, etc.) may be selected to provide a continuousstream of droplets 16, where most of the droplets may contain from oneto several build material particles 22 per droplet. In an example,spraying involves atomizing the one or more dispersions 12 by forcingthe one or more dispersions 12 through a nozzle using a pressuregradient ranging from about 1 atm to about 100 atm.

The droplets 16 impinge and accumulate on a build area platform 26 toform a layer 20 including the build material particles 22 and the liquidagent 24. The build material particles 22 and the liquid agent 24 in thelayer 20 are included in the one or more dispersions 12 prior to thespraying. When the layer 20 is formed by spraying a single dispersion12, all of the build material particles 22 and all of the liquid agent24 in the layer 18 are included in the single dispersion 12 prior to thespraying. When the layer 20 is formed by spraying multiple dispersions,each of the dispersions includes, prior to spraying, some of the buildmaterial particles 22 and some of the liquid agent 24 that end up in thelayer 20. For example, when two dispersions are used, one dispersionincludes some of the build material particles 22 and some of the liquidagent 24 that end up in the layer 20, and the other dispersion includessome other of the build material particles 22 and some other of theliquid agent 24 that end up in the layer 20. The layer 20 may alsoinclude other component(s) of the dispersion(s) 12, such as thedispersant.

The spraying of the one or more dispersions 12 may be accomplished at aspraying temperature. The spraying temperature may range from about 10°C. to about 150° C. In an example, the spraying temperature is about 20°C. In another example, the spraying temperature is about 80° C.

The spraying of the one or more dispersions 12 may be accomplished in asingle pass or in multiple passes. As an example of single passspraying, the desired amount of the dispersion(s) 12 may be sprayedduring the same pass of the applicator(s). As an example of multiplepass spraying, the desired amount of the dispersion(s) may be sprayedover several passes of the applicator(s).

The amount of the dispersion(s) 12 deposited on the build platform 26may depend, at least in part on the desired thickness of the buildmaterial layer 10 to be formed and/or the build material particles 20loading in the dispersion(s) 12. The desired thickness may be achievedby spraying a corresponding amount of the dispersion(s) 12 in a singlepass or over multiple passes (as mentioned above). The speed of thespraying apparatus and the velocity of the spray may be controlled inorder to deposit the desired amount and achieve the desired thickness.

As mentioned above, in examples of the methods 100, 200, 300 disclosedherein, forming a build material layer 10 also includes evaporating theliquid agent 24 from the layer 20. In some examples, evaporating theliquid agent 24 from the layer 20 includes evaporating substantially all(e.g., from about 95 vol % to about 99 vol %) of the liquid agent 24from the layer 20. In other examples, evaporating the liquid agent 24from the layer 20 includes evaporating all (i.e., 100 vol %) of theliquid agent 24 from the layer 20.

In some examples, the liquid agent 24 may evaporate at an ambientprinting temperature. In some of these examples, the ambient printingtemperature may range from about 40° C. to about 50° C. In someexamples, the build area platform 26 is maintained at the ambientprinting temperature, and thus additional heating is not performed inorder to achieve evaporation.

In other examples, the liquid agent 24 may evaporate at a heatingtemperature, which is higher than the ambient printing temperature. Inthese examples, evaporating the liquid agent may include increasing thetemperature to heat the layer 20 to the heating temperature. In someexamples, multiple techniques may be used together to achieve theheating temperature. For example, the temperature of the build areaplatform 26 may be temporarily raised to a first temperature and thenpulse heating may be used to further increase the temperature to thedesired heating temperature. Using a combination of heating techniquesmay enable the liquid agent 24 to be rapidly evaporated.

The heating temperature selected may depend, at least in part, on theliquid agent 24 used, and the build material particles 22 used. In anexample, the heating temperature may be below the melting or softeningpoint of the build material particles 22. In some examples, the heatingtemperature may be at least 5° C., or at least 50° C. below the meltingor softening point of the build material particles 22. In anotherexample, the heating temperature may be below the boiling point of theliquid agent 24. In still another example, the heating temperature maybe equal to or above the boiling point of the liquid agent 24. Asexamples, the heating temperature may range from about 50° C. to about205° C., from about 50° C. to about 100° C., from about 50° C. to about80° C., or from about 80° C. to about 100° C., from about 100° C. toabout 190° C. The ranges provided are some examples, and higher or lowertemperatures may be used.

Heating the layer 20 to the heating temperature may be accomplishedusing any suitable heat source that exposes all of the layer 20 to theheat. Examples of the heat source include a thermal heat source (e.g., aheater (not shown) integrated into the build area platform 26 (which mayinclude sidewalls)) or a radiation source.

Evaporating the liquid agent 24 from the layer 20 forms the buildmaterial layer 10. In some examples, the build material layer 10 that isformed consists of the build material particles 20. In other examples,the build material layer 10 that is formed may include other components,such as the dispersant and/or a residual amount of the liquid agent 24.

In some examples, the build material layer 10 that is formed may have asubstantially uniform thickness ranging from about 1 μm to about 200 μm.In other examples, the build material layer 10 that is formed may have asubstantially uniform thickness ranging from about 1 μm to about 50 μm,from about 1 μm to about 40 μm, from about 10 μm to about 60 μm, or fromabout 10 μm to about 40 μm. It is to be understood that, in each ofthese examples, the substantially uniform thickness values are of thebuild material layer 10 after formation and prior to further processing.

In some examples, the methods 100, 200, 300 further comprise agitatingthe one or more dispersions 12 prior to the spraying of the one or moredispersions 12. The agitating process may be accomplished by anysuitable means, such as shaking, stirring, etc. Agitation may help touniformly disperse the build material particles 20 throughout thedispersion(s) 12 just prior to spraying, which may help the buildmaterial layer 10 that is formed to have a substantially uniformthickness.

It is to be understood that spraying one or more dispersions 12 to forma layer 20 including build material particles 22 and a liquid agent 24,and evaporating the liquid agent 24 from the layer 20 to form a buildmaterial layer 10 may be repeated to form additional build materiallayers. Each of the additional layers may be formed after the previousbuild material layer is patterned (e.g., with the binder agent or themasking agent) and/or exposed to radiated energy (e.g., with a floodenergy source or a laser).

Printing with Binder Agents

Referring now to FIG. 6 , in examples of the method 100 disclosedherein, after the build material layer 10 is formed, a binder agent 28is selectively applied on at least a portion of the build material layer10. In these examples, the formation and patterning of the buildmaterial layers 10 may be repeated to form a 3D intermediate part 30.Binder particles from the binder agent 28 may be cured to temporarilyhold the build material particles 22 of the 3 D intermediate part 30together so that the 3D intermediate part 30 may be moved from the buildarea platform 26 to a separate device (not shown) for heating. Heatingmay remove the binder particles and sinter the build material particles22 of the 3D intermediate part 30 to form the 3D object.

It is to be understood that examples of the method 100 may be used whenthe build material particles 22 include ceramic particles, metalparticles, or a combination thereof. These particles are able towithstand the high temperature heating to remove the binder particlesand to sinter the build material particles 22 to form the 3D object.

Binder Agents

As mentioned above, to bind the build material particles 22 on a layerby layer basis and form a 3D intermediate part 30, a binder agent 28with a polymer binder can be used. In some examples of the method 100,the binder agent 28 consists of a liquid vehicle and the polymer binder.In other examples, the binder agent may include other components.

Polymer Binder

The following discussion relates particularly to the polymer particlesthat can be used as the polymer binder in the binder agent 28. In someexamples, the polymer particles are latex particles. Latex particlesrefer to any polymer (homopolymer, co-polymer, or heteropolymer) that iscapable of being dispersed in an aqueous medium.

The polymer (latex) particles may have several different morphologies.In one example, the polymer particles can include two differentcopolymer compositions, which may be fully separated core-shellpolymers, partially occluded mixtures, or intimately comingled as apolymer solution. In another example, the polymer particles can beindividual spherical particles containing polymer compositions ofhydrophilic (hard) component(s) and/or hydrophobic (soft) component(s)that can be interdispersed. In one example, the interdispersion can beaccording to IPN (interpenetrating networks) although it is contemplatedthat the hydrophilic and hydrophobic components may be interdispersed inother ways. In yet another example, the polymer particles can becomposed of a hydrophobic core surrounded by a continuous ordiscontinuous hydrophilic shell. For example, the particle morphologycan resemble a raspberry, in which a hydrophobic core can be surroundedby several smaller hydrophilic particles that can be attached to thecore. In yet another example, the polymer particles can include 2, 3, or4 or more relatively large polymer particles that can be attached to oneanother or can surround a smaller polymer core. In a further example,the polymer particles can have a single phase morphology that can bepartially occluded, can be multiple-lobed, or can include anycombination of any of the morphologies disclosed herein.

In some examples, the polymer particles can be homopolymers. In otherexamples, the polymer particles can be heteropolymers or copolymers. Inan example, a heteropolymer can include a hydrophobic component and ahydrophilic component. In this example, the heteropolymer can include ahydrophobic component that can include from about 65% to about 99.9% (byweight of the heteropolymer), and a hydrophilic component that caninclude from about 0.1% to about 35% (by weight of the heteropolymer).In one example, the hydrophobic component can have a lower glasstransition temperature than the hydrophilic component.

Examples of monomers that may be used to form the hydrophobic componentof the heteropolymer polymer (latex) particles include C4 to C8 alkylacrylates or methacrylates, styrene, substituted methyl styrenes, polyolacrylates or methacrylates, vinyl monomers, vinyl esters, ethylene,maleate esters, fumarate esters, itaconate esters, or the like. Somespecific example monomers can include, C1 to C20 linear or branchedalkyl (meth)acrylate, alicyclic (meth)acrylate, alkyl acrylate, styrene,methyl styrene, polyol (meth)acrylate, hydroxyethyl (meth)acrylate, or acombination thereof. In one specific class of examples, the polymer(latex) particles can be a styrene (meth)acrylate copolymer. In stillanother example, the polymer (latex) particles can include a copolymerwith copolymerized methyl methacrylate being present at about 50 wt % orgreater, or copolymerized styrene being present at about 50 wt % orgreater. Both can be present, with one or the other at about 50 wt % orgreater in a more specific example.

The term “(meth)acrylate” or “(meth)acrylic acid” or the like refers tomonomers, copolymerized monomers, etc., that can either be acrylate ormethacrylate (or a combination of both), or acrylic acid or methacrylicacid (or a combination of both). In some examples, the terms“(meth)acrylate” and “(meth)acrylic acid” can be used interchangeably,as acrylates and methacrylates are salts and esters of acrylic acid andmethacrylic acid, respectively. Furthermore, mention of one compoundover another can be a function of pH. Furthermore, even if the monomerused to form the polymer was in the form of a (meth)acrylic acid duringpreparation, pH modifications during preparation or subsequently whenadded to an ejectable liquid, such as the binder agent 28, can impactthe nature of the moiety as well (acid form vs. salt or ester form).Thus, a monomer or a moiety of a polymer described as (meth)acrylic acidor as (meth)acrylate should not be read so rigidly as to not considerrelative pH levels, ester chemistry, and other general organic chemistryconcepts.

In still other examples, the polymer (latex) particles in the binderagent include polymerized monomers of vinyl chloride, vinylidenechloride, vinylbenzyl chloride, vinyl ester, styrene, ethylene, maleateesters, fumarate esters, itaconate esters, α-methyl styrene, p-methylstyrene, methyl methacrylate, hexyl acrylate, hexyl methacrylate,hydroxyethyl acrylate, butyl acrylate, butyl methacrylate, ethylacrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate,2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, octadecyl acrylate,octadecyl methacrylate, stearyl methacrylate, 2-phenoxyethylmethacrylate, isobornyl acrylate, tetrahydrofurfuryl acrylate, benzylmethacrylate, benzyl acrylate, ethoxylated nonyl phenol methacrylate,isobornyl methacrylate, ethylene glycol dimethacrylate, diethyleneglycol dimethacrylate, triethylene glycol dimethacrylate, cyclohexylmethacrylate, trimethyl cyclohexyl methacrylate, t-butyl methacrylate,n-octyl methacrylate, lauryl acrylate, lauryl methacrylate, trydecylmethacrylate, alkoxylated tetrahydrofurfuryl acrylate, isodecylacrylate, dimethyl maleate, dioctyl maleate, acetoacetoxyethylmethacrylate, diacetone acrylamide, N-vinyl imidazole, N-vinylcarbazole,N-Vinyl-caprolactam, pentaerythritol tri-acrylate, pentaerythritoltetra-acrylate, pentaerythritol tri-methacrylate, pentaerythritoltetra-methacrylate, glycidol acrylate, glycidol methacrylate,tetrahydrofuryl acrylate, tetrahydrofuryl methacrylate, diacetoneacrylamide, t-butyl acrylamide, divinylbenzene, 1,3-butadiene,acrylonitrile, methacrylonitrile, combinations thereof, derivativesthereof, or mixtures thereof. These monomers include low glasstransition temperature (Tg) monomers that can be used to form thehydrophobic component of a heteropolymer.

In some examples, a composition of the polymer (latex) particles caninclude acidic monomer(s). In some examples, the acidic monomer contentcan range from 0.1 wt % to 5 wt %, from 0.5 wt % to 4 wt %, or from 1 wt% to 2.5 wt % of the polymer particles with the remainder of the polymerparticles being composed of non-acidic monomers. Example acidic monomerscan include acrylic acid, methacrylic acid, ethacrylic acid,dimethylacrylic acid, maleic anhydride, maleic acid, vinylsulfonate,cyanoacrylic acid, vinylacetic acid, allylacetic acid, crotonoic acid,fumaric acid, itaconic acid, sorbic acid, angelic acid, cinnamic acid,styrylacrylic acid, citraconic acid, glutaconic acid, aconitic acid,phenylacrylic acid, acryloxypropionic acid, aconitic acid, phenylacrylicacid, acryloxypropionic acid, vinylbenzoic acid, N-vinylsuccinamidicacid, mesaconic acid, methacroylalanine, acryloylhydroxyglycine,sulfoethyl methacrylic acid, sulfopropyl acrylic acid, styrene sulfonicacid, sulfoethylacrylic acid, 2-methacryloyloxymethane-1-sulfonic acid,3-methacryoyloxypropane-1-sulfonic acid, 3-(vinyloxy)propane-1-sulfonicacid, ethylenesulfonic acid, vinyl sulfuric acid, 4-vinylphenyl sulfuricacid, ethylene phosphonic acid, vinyl phosphoric acid, vinyl benzoicacid, 2-acrylamido-2-methyl-1-propanesulfonic acid, combinationsthereof, derivatives thereof, or mixtures thereof. These acidic monomersare higher Tg hydrophilic monomers, than the low Tg monomers above, andcan be used to form the hydrophilic component of a heteropolymer. Otherexamples of high Tg hydrophilic monomers can include acrylamide,methacrylamide, monohydroxylated monomers, monoethoxylated monomers,polyhydroxylated monomers, or polyethoxylated monomers.

In an example, the selected monomer(s) can be polymerized to form apolymer, heteropolymer, or copolymer with a co-polymerizable dispersingagent. The co-polymerizable dispersing agent can be a polyoxyethylenecompound, such as a HITENOL® compound (Montello Inc.) e.g.,polyoxyethylene alkylphenyl ether ammonium sulfate, sodiumpolyoxyethylene alkylether sulfuric ester, polyoxyethylene styrenatedphenyl ether ammonium sulfate, or mixtures thereof.

Any suitable polymerization process can be used to form the polymerparticles. In some examples, an aqueous dispersion of latex particlescan be produced by emulsion polymerization or co-polymerization of anyof the above monomers.

In one example, the polymer (latex) particles can be prepared bypolymerizing high Tg hydrophilic monomers to form the high Tghydrophilic component and attaching the high Tg hydrophilic componentonto the surface of the low Tg hydrophobic component. In anotherexample, the polymer (latex) particles can be prepared by polymerizingthe low Tg hydrophobic monomers and the high Tg hydrophilic monomers ata ratio of the low Tg hydrophobic monomers to the high Tg hydrophilicmonomers that ranges from 5:95 to 30:70. In this example, the low Tghydrophobic monomers can dissolve in the high Tg hydrophilic monomers.In yet another example, the polymer (latex) particles can be prepared bypolymerizing the low Tg hydrophobic monomers, then adding the high Tghydrophilic monomers. In this example, the polymerization process cancause a higher concentration of the high Tg hydrophilic monomers topolymerize at or near the surface of the low Tg hydrophobic component.In still another example, the polymer (latex) particles can be preparedby copolymerizing the low Tg hydrophobic monomers and the high Tghydrophilic monomers, then adding additional high Tg hydrophilicmonomers. In this example, the copolymerization process can cause ahigher concentration of the high Tg hydrophilic monomers to copolymerizeat or near the surface of the low Tg hydrophobic component.

Other suitable techniques, specifically for generating a core-shellstructure, can include grafting a hydrophilic shell onto the surface ofa hydrophobic core, copolymerizing hydrophobic and hydrophilic monomersusing ratios that lead to a more hydrophilic shell, adding hydrophilicmonomer (or excess hydrophilic monomer) toward the end of thecopolymerization process so there is a higher concentration ofhydrophilic monomer copolymerized at or near the surface, or any othermethod can be used to generate a more hydrophilic shell relative to thecore.

In one specific example, the low Tg hydrophobic monomers can be selectedfrom the group consisting of C4 to C8 alkyl acrylate monomers, C4 to C8alkyl methacrylate monomers, styrene monomers, substituted methylstyrene monomers, vinyl monomers, vinyl ester monomers, and combinationsthereof; and the high Tg hydrophilic monomers can be selected fromacidic monomers, unsubstituted amide monomers, alcoholic acrylatemonomers, alcoholic methacrylate monomers, C1 to C2 alkyl acrylatemonomers, C1 to C2 alkyl methacrylate monomers, and combinationsthereof. The resulting polymer latex particles can exhibit a core-shellstructure, a mixed or intermingled polymeric structure, or some othermorphology.

In some examples, the polymer (latex) particles can have a weightaverage molecular weight (Mw, g/mol) that can range from about 5,000 Mwto about 2,000,000 Mw. In yet other examples, the weight averagemolecular weight can range from about 100,000 Mw to about 1,000,000 Mw,from about 100,000 Mw to about 500,000 Mw, from about 150,000 Mw toabout 300,000 Mw, or from about 50,000 Mw to about 250,000 Mw. Weightaverage molecular weight (Mw) can be measured by Gel PermeationChromatography with polystyrene standard.

In some examples, the polymer (latex) particles can be latent and can beactivated by heat (which may be applied iteratively during 3Dintermediate part formation or after 3D intermediate part formation). Inthese instances, the activation temperature can correspond to theminimum film formation temperature (MFFT) or a glass transitiontemperature (T_(g)) which can be greater than room temperature (e.g.,ranging about 18° C. to about 22° C.). In one example, the polymer(latex) particles can have a MFFT or T_(g) that can be at least about15° C. greater than ambient temperature. In another example, the MFFT orthe T_(g) of the bulk material (e.g., the more hydrophobic portion) ofthe polymer (latex) particles can range from about 25° C. to about 200°C. In another example, the polymer (latex) particles can have a MFFT orT_(g) ranging from about 40° C. to about 120° C. In yet another example,the polymer (latex) particles can have a MFFT or T_(g) ranging fromabout 0° C. to about 150° C. In a further example, the polymer latexparticles can have a T_(g) that can range from about −20° C. to about130° C., or in another example from about 60° C. to about 105° C. At atemperature above the MFFT or the T_(g) of a latent latex polymerparticle, the polymer particles can coalesce and can bind materials,such as the build material particles 22.

The polymer (latex) particles can have a particle size that can bejetted via thermal ejection or printing, piezoelectric ejection orprinting, drop-on-demand ejection or printing, continuous ejection orprinting, etc. In an example, the particle size of the polymer (latex)particles can range from about 1 nm to about 400 nm. In yet otherexamples, a particle size of the polymer particles can range from about10 nm to about 300 nm, from about 50 nm to about 250 nm, from about 100nm to about 250 nm, or from about 25 nm to about 250 nm. In someexamples, the polymer particles can have a particle size that can bejetted via thermal ejection or printing, piezoelectric ejection orprinting, drop-on-demand ejection or printing, continuous ejection orprinting, etc. In these examples, the particle size of the polymerparticles be about 100 nm or more.

In some examples, the polymer (latex) particles have a glass transitiontemperature higher than 60° C. and an average particle size of 1 nm ormore.

Other examples of the polymer (latex) particles include polyvinylalcohol, polyvinylpyrrolidone, and combinations thereof. Examples ofpolyvinyl alcohol include low weight average molecular weight polyvinylalcohols (e.g., from about 13,000 to about 50,000), such as SELVOL™ PVOH17 from Sekisui. Examples of polyvinylpyrrolidones include low weightaverage molecular weight polyvinylpyrrolidones (e.g., from about 15,000to about 19,000), such as LUVITEC™ K 17 from BASF Corp.

It is to be understood that the examples of the polymer binder providedare some examples, and that other polymer binders may be used.

In an example, the polymer binder is present in the binder agent 28 inan amount ranging from about 1 wt % active to about 40 wt % active basedon a total weight of the binder agent 28. In other examples, the polymerbinder may be present in the binder agent 28 in an amount ranging fromabout 2 wt % active to about 30 wt % active, from about 5 wt % active toabout 30 wt % active, from about 12 wt % active to about 22 wt % active,from about 15 wt % active to about 20 wt % active, from about 10 wt %active to about 20 wt % active, or from about 6 wt % active to about 18wt % active, based on the total weight of binder agent 28.

Liquid Vehicles

In addition to the polymer binder, the binder agent 28 may also includea liquid vehicle. As used herein, the term “liquid vehicle” may refer tothe liquid to which the polymer binder is added to form the binder agent28.

In some examples, the liquid vehicle may make up about 60 wt % to about99 wt % of the binder agent 28. In other examples, the liquid vehiclemay be included in the binder agent 28 in an amount ranging from about60 wt % to about 95 wt %, from about 60 wt % to about 90 wt %, fromabout 60 wt % to about 85 wt %, from about 60 wt % to about 80 wt %,from about 75 wt % to about 90 wt %, or from about 70 wt % to about 80wt %, based on a total weight of the binder agent 28.

The liquid vehicle of the binder agent 28 may include water,co-solvent(s), humectant(s), surfactant(s), dispersing agent(s),antimicrobial agent(s), viscosity modifier(s), pH adjuster(s), chelatingagent(s), and the like. In one example, water can be present in anamount ranging from about 30 wt % to 100 wt % of the vehiclecomponent—excluding the polymer binder—based on a total weight of theliquid vehicle. In other examples, the water can be present in an amountranging from about 60 wt % to about 95 wt %, from about 75 wt % to 100wt %, or from about 80 wt % to about 99 wt %, based on a total weight ofthe liquid vehicle.

The co-solvent can be present at from about 0.5 wt % to about 50 wt % inthe liquid vehicle, based on a total weight of the binder agent 28. Insome examples, the co-solvent can be a high boiling point solvent, whichcan have a boiling point of at least about 110° C. Example co-solventscan include aliphatic alcohols, aromatic alcohols, alkyl diols, glycolethers, polyglycol ethers, lactams, caprolactams, formamides,acetamides, long chain alcohols, and combinations thereof. For example,the co-solvent can include aliphatic alcohols with a —CH₂OH group,secondary aliphatic alcohols, 1,2-alcohols, 1,3-alcohols, 1,5-alcohols,ethylene glycol alkyl ethers, propylene glycol alkyl ethers, C6 to C12homologs of polyethylene glycol alkyl ethers, N-alkyl caprolactams,unsubstituted caprolactams, both substituted and unsubstitutedformamides, both substituted and unsubstituted acetamides, combinationsthereof, and the like. Other example organic co-solvents can includepropyleneglycol ether, dipropyleneglycol monomethyl ether,dipropyleneglycol monopropyl ether, dipropyleneglycol monobutyl ether,tripropyleneglycol monomethyl ether, tripropyleneglycol monobutyl ether,dipropyleneglycol monophenyl ether, 2-pyrrolidone,2-methyl-1,3-propanediol (MP-diol), and combinations thereof.

The liquid vehicle may also include humectant(s). In an example, thetotal amount of the humectant(s) present in the binder agent 28 rangesfrom about 3 wt % active to about 10 wt % active, based on the totalweight of the binder agent 28. An example of a suitable humectant isethoxylated glycerin having the following formula:

in which the total of a+b+c ranges from about 5 to about 60, or in otherexamples, from about 20 to about 30. An example of the ethoxylatedglycerin is LIPONIC® EG-1 (LEG-1, glycereth-26, a+b+c=26, available fromLipo Chemicals).

If a surfactant/dispersing agent is included, examples can includeSURFYNOL® SEF (a self-emulsifiable wetting agent based on acetylenicdiol chemistry), SURFYNOL® 104 (a non-ionic wetting agent based onacetylenic diol chemistry), or SURFYNOL ® 440 (an ethoxylated low-foamwetting agent) (all available from Evonik Industries AG, Germany);TERGITOL® TMN-6 (a branched secondary alcohol ethoxylate, non-ionicsurfactant), TERGITOL® 15-S-5 or TERGITOL® 15-S-7 (each of which is asecondary alcohol ethoxylate, non-ionic surfactant), or DOWFAX® 2A1 orDOWFAX® 8390 (each of which is an alkyldiphenyloxide disulfonate,available from Dow, USA); CAPSTONE® FS-35 (non-ionic fluorosurfactantfrom DuPont, USA) or a combination thereof. The surfactant orcombinations of surfactants may be present in the binder agent 28 in anamount ranging from about 0.1 wt % active to about 5 wt % active basedon the total weight of the binder agent, and in some examples, may bepresent at from about 0.5 wt % active to about 2 wt % active.

With respect to antimicrobial agents, any compound suitable to inhibitthe growth of harmful microorganisms can be included. These additivesmay be biocides, fungicides, and other microbial agents. Examples ofsuitable antimicrobials can include NUOSEPT® (Troy, Corp.), UCARCIDE™,KORDEK™, ROCIMA™, KATHON™ (all available from The Dow Chemical Co.),VANCIDE® (R.T. Vanderbilt Co.), PROXEL® (Arch Chemicals), ACTICIDE® B20and ACTICIDE® M20 and ACTICIDE® MBL (blends of2-methyl-4-isothiazolin-3-one (MIT), 1,2-benzisothiazolin-3-one (BIT),and Bronopol (Thor Chemicals); AXIDE™ (Planet Chemical); NIPACIDE™(Clariant), etc. In an example, the binder agent may include a totalamount of antimicrobials that ranges from about 0.0001 wt % active toabout 1 wt % active.

Chelating agents (or sequestering agents), such as EDTA (ethylenediamine tetra acetic acid), may be included to eliminate the deleteriouseffects of heavy metal impurities. Whether a single chelating agent isused or a combination of chelating agents is used, the total amount ofchelating agent(s) in the binder agent 28 may range from greater than 0wt % active to about 2 wt % active based on the total weight of thebinder agent 28.

Viscosity modifiers and buffers may also be present, as well as otheradditives to modify properties of the binder agent 28. For example,buffer solutions may be used to control the pH of the binder agent 28.

In some examples, the liquid vehicle may also include from about 0.1 wt% active to about 1 wt % active of an anti-kogation agent, based on atotal weight of the binder agent. Kogation refers to the deposit ofdried solids on a printhead. An anti-kogation agent can be included toprevent the buildup of dried solids on the printhead. Examples ofsuitable anti-kogation agents can include oleth-3-phosphate(commercially available as CRODAFOS™ O3A or CRODAFOS™ N-3 acid), dextran500k, CRODAFOS™ HCE (phosphate-ester from Croda Int.), CRODAFOS® N10(oleth-10-phosphate from Croda Int.), or DISPERSOGEN® LFH (polymericdispersing agent with aromatic anchoring groups, acid form, anionic,from Clariant), etc.

Printing Method

In examples of the method 100, a build material layer 10 may be formedas described herein in reference to FIG. 5 .

As shown in FIG. 6 , after the formation of the build material layer 10,the binder agent 28 may be selectively applied on at least a portion 34of the build material layer 10. The binder agent 28 may be ejected ontothe build material layer 10 from an applicator 32 (such as a thermalinkjet printhead or a piezoelectric inkjet printhead), for example, toselectively pattern the build material layer 10. The location(s) of theselectively applied binder agent 28 can be selected to correspond with alayer of a 3D printed object, such as from a 3D object model or computermodel.

Heat may be applied, such as from a heat source, to each layer 10 (or tothe group of layers 10, 10′, 10″) to remove water from the binder agent28 throughout the patterning process. This temperature is 100° C. orless. In one example, heat can be applied from overhead, e.g., prior toformation of the next build material layer 10, or after multiple layersare patterned, etc.), and/or can be provided by the build area platform26 from beneath the build material layer(s) 10.

After individual build material layers 10, 10′, 10″ are patterned withbinder agent 28, the build area platform 26 can be dropped a distance,which may correspond to at least the thickness of a patterned layer inone example, so that another build material layer 10′, 10″ may be formedthereon and patterned with the binder agent 28, etc. The process can berepeated on a layer by layer basis until all of the desired layers arepatterned in accordance with a 3D object model.

The applicator 32 can deposit the binder agent 28 in the layers 10, 10′,10″ in a pattern that corresponds to the layers of the 3D object, andcan be used to form a 3D intermediate part 30 in any orientation. Forexample, the 3D intermediate part 30 can be printed from bottom to top,top to bottom, on its side, at an angle, or any other orientation. Theorientation of the 3D intermediate part 30 can also be formed in anyorientation relative to the layering of the build material particles 22.For example, the 3D intermediate part 30 can be formed in an invertedorientation or on its side relative to the layering of the buildmaterial. The orientation of build or the orientation of the 3Dintermediate part 30 within the build material 22 can be selected inadvance or even by the user at the time of printing, for example.

After all of the desired layers 10, 10′, 10″ of build material 22 arepatterned with the binder agent 28, heating all of the individuallypatterned layers is performed. This heating is performed at atemperature ranging from about 120° C. to about 200° C. At thistemperature range, heating coalesces the polymer (latex) particles toform a strong polymer film. As such, heating all of the individuallypatterned layers forms a polymeric network among the build materialparticles 22 in the patterned portions 34 of all of the individuallypatterned layers, thereby forming a 3D intermediate object 30. The nowcured portions form the 3D intermediate object 30, and any non-patternedbuild material 22 surrounding the 3D intermediate object 30 remainnon-cured.

Heating may occur after patterning of all of the layers 10, 10′, 10″,and thus the time frames can vary depending on size of the 3Dintermediate part 30. For example, large objects with a smaller surfaceto volume ratio may take longer to drive off enough liquid to stabilizethe 3D intermediate part, than a smaller object with a larger surface tovolume ratio. That stated, time frames for heating the patterned layerscan be from about 10 minutes to about 8 hours, or from about 30 minutesto about 3 hours.

The 3D intermediate part 30, in this example, includes a 3D objectformed of cured/solidified 3D intermediate part layers, which includeboth build material particles 22 and a network of polymeric particlesholding the build material particles 22 together. The 3D intermediatepart 30 that is formed is stable enough to be moved to an oven (or otherheating device) suitable for de-binding and sintering e.g., annealing,melting, fusing, or the like.

In one example, the heating can be at a temperature ranging from about500° C. to about 3,500° C. At lower temperatures within the range, thenetwork of the polymer particles can thermally degrade, therebydebinding the 3D intermediate part 30, and at the higher temperatureswithin the range, the build material particles 22 are sintered together.In another example, the de-binding and sintering temperatures can be inthe range of from about 600° C. to about 1,500° C., or from about 800°C. to about 1,200° C. The de-binding temperature range can vary,depending on the composition of the network. The sintering temperaturerange can vary, depending on the build material particles 22. In oneexample, the sintering temperature can range from about 10° C. below themelting temperature of the build material particles 22 to about 50° C.below the melting temperature of the build material particles 22. Inanother example, the sintering temperature can range from about 100° C.below the melting temperature of the build material particles 22 toabout 200° C. below the melting temperature of the build materialparticles 22. The sintering temperature can also depend upon theparticle size and period of time that heating occurs, e.g., at a hightemperature for a sufficient time to cause particle surfaces to becomephysically merged or composited together. For example, a sinteringtemperature for stainless steel can be about 1,400° C. and an example ofa sintering temperature for aluminum or aluminum alloys can range fromabout 550° C. to about 620° C. Temperatures outside of these ranges canbe used as determined on a case by case basis.

When the build material particles 22 include metal particles, theheating device can include an inert or low-reactivity atmosphere toavoid oxidation of the metal particles. In one example, the inert orlow-reactivity atmosphere can be oxygen-free and can include a noblegas, an inert gas, a low-reactivity gas, or combination thereof. Forexample, the inert or low-reactivity atmosphere can include a noble gas,an inert gas, or low-reactivity gas selected from argon, nitrogen,helium, neon, krypton, xenon, radon, hydrogen, or a combination thereof.Upon removal of the sintered 3D object from the oven and cooling (orannealing by controlling the cool down rate in the oven), the sintered3D object can be treated or polished, such as by sand blasting, beadblasting, air jetting, tumble finishing such as barrel finishing,vibratory finishing, or a combination thereof. Tumble or vibratoryfinishing techniques can be performed wet (involving liquid lubricants,cleaners, or abrasives) or dry.

Printing with Masking Agents

Referring now to FIG. 7 , in examples of the method 200 disclosedherein, after the build material layer 10 is formed, a masking agent36N, 36P is selectively applied on at least a portion 34 of the buildmaterial layer 10, and the build material layer 10 is exposed toradiated energy. When a positive masking agent 36P is used, thepatterned region 34 of the build material layer 10 is sintered or fusedto become a layer 40 of a 3D object. When a negative masking agent 36Nis used, the non-patterned region 38 of the build material layer 10 issintered or fused to become a layer 40′ of a 3D object. In either ofthese examples, the formation, patterning, and exposing of buildmaterial layers 10 to radiation may be repeated to form the 3D object.

Examples of the method 200 may be used when the build material particles22 include polymer particles, ceramic particles, metal particles, or acombination thereof.

Positive Masking Agents

As mentioned above, the positive masking agent 36P may be selectivelyapplied on at least a portion 34 of a build material layer 10. In theseexamples, the positive masking agent 36P may absorb radiation to whichthe build material layer 10 is exposed and cause the build materialparticles 22 in the at least portion 34 to sinter or fuse.

In some examples of the method 200, the positive masking agent 36Pconsists of a liquid vehicle and an energy absorber. In other examples,the positive masking agent may include other components.

Energy Absorber

The energy absorber is capable of absorbing radiated energy andconverting the absorbed radiated energy to thermal energy. The thermalenergy sufficiently raises the temperature of the build materialparticles 22 that are in contact with the energy absorber so that thosebuild material particles 22 sinter or fuse to become a layer 40 of a 3Dobject, while the build material particles 22 that are not in contactwith the energy absorber do not sinter or fuse. The energy absorber usedmay depend, at least in part, on the build material particles 22 used.

As used herein “absorption” means that at least 80% of radiation havingwavelengths within the specified range is absorbed. Also as used herein,“transparency” means that 25% or less of radiation having wavelengthswithin the specified range is absorbed.

In some examples, the energy absorber may be an infrared light absorbingcolorant. In an example, the energy absorber is a near-infrared lightabsorber. Any near-infrared colorants, e.g., those produced byFabricolor, Eastman Kodak, or BASF, Yamamoto, may be used in thepositive masking agent 36P. As one example, the positive masking agent36P may be a printing liquid formulation including carbon black as theenergy absorber. Examples of this printing liquid formulation arecommercially known as CM997A, 516458, C18928, C93848, C93808, or thelike, all of which are available from HP Inc.

As another example, the positive masking agent 36P may be a printingliquid formulation including near-infrared absorbing dyes as the energyabsorber. Examples of this printing liquid formulation are described inU.S. Pat. No. 9,133,344, incorporated herein by reference in itsentirety. Some examples of the near-infrared absorbing dye arewater-soluble near-infrared absorbing dyes selected from the groupconsisting of:

and mixtures thereof. In the above structures, M can be a divalent metalatom (e.g., copper, etc.) or can have OSO₃Na axial groups filling anyunfilled valencies if the metal is more than divalent (e.g., indium,etc.), R can be hydrogen or any C₁-C₈ alkyl group (including substitutedalkyl and unsubstituted alkyl), and Z can be a counterion such that theoverall charge of the near-infrared absorbing dye is neutral. Forexample, the counterion can be sodium, lithium, potassium, NH₄ ⁺, etc.

Some other examples of the near-infrared absorbing dye are hydrophobicnear-infrared absorbing dyes selected from the group consisting of:

and mixtures thereof. For the hydrophobic near-infrared absorbing dyes,M can be a divalent metal atom (e.g., copper, etc.) or can include ametal that has Cl, Br, or OR′ (R′═H, CH₃, COCH₃, COCH₂COOCH₃,COCH₂COCH₃) axial groups filling any unfilled valencies if the metal ismore than divalent, and R can be hydrogen or any C₁-C₈ alkyl group(including substituted alkyl and unsubstituted alkyl).

Other near-infrared absorbing dyes or pigments may be used. Someexamples include anthroquinone dyes or pigments, metal dithiolene dyesor pigments, cyanine dyes or pigments, perylenediimide dyes or pigments,croconium dyes or pigments, pyrilium or thiopyrilium dyes or pigments,boron-dipyrromethene dyes or pigments, or aza-boron-dipyrromethene dyesor pigments.

Anthroquinone dyes or pigments and metal (e.g., nickel) dithiolene dyesor pigments may have the following structures, respectively:

where R in the anthroquinone dyes or pigments may be hydrogen or anyC₁-C₈ alkyl group (including substituted alkyl and unsubstituted alkyl),and R in the dithiolene may be hydrogen, COOH, SO₃, NH₂, any C₁-C₈ alkylgroup (including substituted alkyl and unsubstituted alkyl), or thelike.

Cyanine dyes or pigments and perylenediimide dyes or pigments may havethe following structures, respectively:

where R in the perylenediimide dyes or pigments may be hydrogen or anyC₁-C₈ alkyl group (including substituted alkyl and unsubstituted alkyl).

Croconium dyes or pigments and pyrilium or thiopyrilium dyes or pigments

may have the following structures, respectively:

Boron-dipyrromethene dyes or pigments and aza-boron-dipyrromethene dyesor pigments may have the following structures, respectively:

In other examples, the energy absorber may have absorption atwavelengths ranging from 800 nm to 4000 nm and transparency atwavelengths ranging from 400 nm to 780 nm. The absorption of this energyabsorber is the result of plasmonic resonance effects. Electronsassociated with the atoms of the energy absorber may be collectivelyexcited by radiation, which results in collective oscillation of theelectrons. The wavelengths that can excite and oscillate these electronscollectively are dependent on the number of electrons present in theenergy absorber particles, which in turn is dependent on the size of theenergy absorber particles. The amount of energy that can collectivelyoscillate the particle's electrons is low enough that very smallparticles (e.g., 1-100 nm) may absorb radiation with wavelengths severaltimes (e.g., from 8 to 800 or more times) the size of the particles. Theuse of these particles allows the positive masking agent to be inkjetjettable as well as electromagnetically selective (e.g., havingabsorption at wavelengths ranging from 800 nm to 4000 nm andtransparency at wavelengths ranging from 400 nm to 780 nm).

In an example, this energy absorber has an average particle diameter(e.g., volume-weighted mean diameter) ranging from greater than 0 nm toless than 220 nm. In another example, the energy absorber has an averageparticle diameter ranging from greater than 0 nm to 120 nm. In a stillanother example, the energy absorber has an average particle diameterranging from about 10 nm to about 200 nm.

In an example, this energy absorber is an inorganic pigment. Examples ofsuitable inorganic pigments include lanthanum hexaboride (LaB₆),tungsten bronzes (A_(x)WO₃), indium tin oxide (In₂O₃:SnO₂, ITO),antimony tin oxide (Sb₂O₃:SnO₂, ATO), titanium nitride (TiN), aluminumzinc oxide (AZO), ruthenium oxide (RuO₂), silver (Ag), gold (Au),platinum (Pt), iron pyroxenes (A_(x)Fe_(y)Si₂O₆ wherein A is Ca or Mg,x=1.5-1.9, and y=0.1-0.5), modified iron phosphates (A_(x)Fe_(y)PO₄),modified copper phosphates (A_(x)Cu_(y)PO_(z)), and modified copperpyrophosphates (A_(x)Cu_(y)O₂O₇). Tungsten bronzes may be alkali dopedtungsten oxides. Examples of suitable alkali dopants (i.e., A inA_(x)WO₃) may be cesium, sodium, potassium, or rubidium. In an example,the alkali doped tungsten oxide may be doped in an amount ranging fromgreater than 0 mol % to about 0.33 mol % based on the total mol % of thealkali doped tungsten oxide. Suitable modified iron phosphates(A_(x)Fe_(y)PO) may include copper iron phosphate (A═Cu, x=0.1-0.5, andy=0.5-0.9), magnesium iron phosphate (A═Mg, x=0.1-0.5, and y=0.5-0.9),and zinc iron phosphate (A═Zn, x=0.1-0.5, and y=0.5-0.9). For themodified iron phosphates, it is to be understood that the number ofphosphates may change based on the charge balance with the cations.Suitable modified copper pyrophosphates (A_(x)Cu_(y)P₂O₇) include ironcopper pyrophosphate (A═Fe, x=0-2, and y=0-2), magnesium copperpyrophosphate (A═Mg, x=0-2, and y=0-2), and zinc copper pyrophosphate(A═Zn, x=0-2, and y=0-2). Combinations of the inorganic pigments mayalso be used.

It is to be understood that the examples of the energy absorber providedare some examples, and that other energy absorbers may be used.

The amount of the energy absorber that is present in the positivemasking agent 36P ranges from greater than 0 wt % active to about 40 wt% active based on the total weight of the positive masking agent 36P. Inother examples, the amount of the energy absorber in the positivemasking agent 36P ranges from about 0.3 wt % active to 30 wt % active,from about 1 wt % active to about 20 wt % active, from about 1.0 wt %active up to about 10.0 wt % active, or from greater than 4.0 wt %active up to about 15.0 wt % active. It is believed that these energyabsorber loadings provide a balance between the positive masking agent36P having jetting reliability and heat and/or radiation absorbanceefficiency.

PMA Vehicles

In addition to the energy absorber, the positive masking agent 36P mayalso include a liquid vehicle (i.e., a PMA vehicle) in which the energyabsorber is dispersed or dissolved to form the positive masking agent36P.

In some examples, the PMA vehicle may make up about 60 wt % to about 99wt % of the positive masking agent 36P. In other examples, the PMAvehicle may be included in the positive masking agent 36P in an amountranging from about 60 wt % to about 95 wt %, from about 60 wt % to about90 wt %, from about 60 wt % to about 85 wt %, from about 60 wt % toabout 80 wt %, from about 75 wt % to about 90 wt %, or from about 70 wt% to about 80 wt %, based on a total weight of the positive maskingagent 36P.

The solvent of the positive masking agent 36P may be water or anon-aqueous solvent (e.g., ethanol, acetone, n-methyl pyrrolidone,aliphatic hydrocarbons, etc.). In some examples, the positive maskingagent 36P consists of the energy absorber and the solvent (without othercomponents). In these examples, the solvent makes up the balance of thepositive masking agent 36P. In other examples, the PMA vehicle mayinclude other components. Examples of other suitable positive maskingagent components include dispersant(s), silane coupling agent(s),co-solvent(s), humectant(s), surfactant(s), antimicrobial agent(s),anti-kogation agent(s), chelating agent(s), viscosity modifier(s), pHadjuster(s), and the like.

In some examples, the PMA vehicle of the positive masking agent 36P maybe similar to the liquid vehicle of the binder agent 28. As such, thePMA vehicle of the positive masking agent 36P may include any of thecomponents described above in reference to the liquid vehicle of thebinder agent 28 in any of the amount described above (with the amount(s)being based on the total weight of the positive masking agent 36P ratherthan the total weight of the binder agent 28).

When the energy absorber is an inorganic pigment (having absorption atwavelengths ranging from 800 nm to 4000 nm and transparency atwavelengths ranging from 400 nm to 780 nm), the PMA vehicle may alsoinclude dispersant(s) and/or silane coupling agent(s).

Examples of suitable dispersants include polymer or small moleculedispersants, charged groups attached to the energy absorber surface, orother suitable dispersants. Some specific examples of suitabledispersants include a water-soluble acrylic acid polymer (e.g.,CARBOSPERSE® K7028 available from Lubrizol), water-solublestyrene-acrylic acid copolymers/resins (e.g., JONCRYL® 296, JONCRYL®671, JONCRYL® 678, JONCRYL® 680, JONCRYL® 683, JONCRYL® 690, etc.available from BASF Corp.), a high molecular weight block copolymer withpigment affinic groups (e.g., DISPERBYK®-190 available BYK Additives andInstruments), or water-soluble styrene-maleic anhydridecopolymers/resins. Whether a single dispersant is used or a combinationof dispersants is used, the total amount of dispersant(s) in thepositive masking agent 36P may range from about 10 wt % to about 200 wt% based on the weight of the energy absorber in the positive maskingagent 36P.

A silane coupling agent may also be added to the positive masking agent36P to help bond the organic and inorganic materials. Examples ofsuitable silane coupling agents include the SILQUEST® A seriesmanufactured by Momentive. Whether a single silane coupling agent isused or a combination of silane coupling agents is used, the totalamount of silane coupling agent(s) in the positive masking agent mayrange from about 0.1 wt % active to about 50 wt % active based on theweight of the energy absorber in the positive masking agent 36P. Inother examples, the total amount of silane coupling agent(s) in thepositive masking agent 36P ranges from about 1 wt % active to about 30wt % active or from about 2.5 wt % active to about 25 wt % active, basedon the weight of the energy absorber.

Printing Methods

In examples of the method 200, a build material layer 10 may be formedas described herein in reference to FIG. 5 .

After the formation of the build material layer 10, and prior to furtherprocessing, the build material layer 10 may be exposed to pre-heating.In some of these examples, the heating to evaporate the liquid agent mayalso pre-heat the build material layer. As such, pre-heating may beaccomplished in a manner similar to the heating to evaporate the liquidagent 24 described above.

After the build material layer 10 is formed, and in some instances ispre-heated, the positive masking agent 36P is selectively applied on atleast a portion 34 of the build material layer 10. In some examples ofthe method 200 disclosed herein, prior to the selectively applying ofthe masking agent (e.g., 36P), the build material layer has asubstantially uniform thickness ranging from about 1 μm to about 200 μm.

The volume of the positive masking agent 36P that is applied per unit ofthe build material particles 22 in the patterned portion 34 may besufficient to absorb and convert enough radiated energy so that thebuild material particles 22 in the patterned portion 34 will sinter orfuse. The volume of the positive masking agent 36P that is applied perunit of the build material particles 22 may depend, at least in part, onthe energy absorber used, the energy absorber loading in the positivemasking agent 36P, and the build material particles 22 used.

The positive masking agent 36P may be dispensed from an applicator 32.The applicator 32 may be a thermal inkjet printhead, a piezoelectricprinthead, a continuous inkjet printhead, etc., and the selectiveapplication of the positive masking agent 36P may be accomplished bythermal inkjet printing, piezo electric inkjet printing, continuousinkjet printing, etc.

After the positive masking agent 36P is selectively applied in thedesired portion(s) 34 of the build material layer, the entire buildmaterial layer is exposed to radiated energy.

In some examples of the method 200, the exposing of the build materiallayer 10 to radiated energy is accomplished using a flood energy sourceselected from the group consisting of an infrared lamp, an array ofinfrared emitters, a pulse gas discharge lamp, an array of fiber lasers,a semiconductor laser, a gas laser, an array of the semiconductorlasers, or an array of the gas lasers. One specific example of the pulsegas discharge lamp is a xenon pulse lamp.

The length of time the radiated energy is applied for, or energyexposure time, may be dependent, for example, on one or more of:characteristics of the flood energy source; characteristics of the buildmaterial particles; and/or characteristics of the positive maskingagent.

The positive masking agent 36B enhances the absorption of the radiatedenergy, converts the absorbed radiated energy to thermal energy, andpromotes the transfer of the thermal heat to the build materialparticles 22 in contact therewith. In an example, the positive maskingagent 36B sufficiently elevates the temperature of the build materialparticles 22 in contact therewith so that those build material particles22 sinter or fuse. The application of the radiated energy forms a layerof the 3D object.

In some examples, the radiated energy has a wavelength ranging from 800nm to 4000 nm, or from 800 nm to 1400 nm, or from 800 nm to 1200 nm.Radiated energy having wavelengths within the provided ranges may beabsorbed (e.g., 80% or more of the applied radiation is absorbed) by thepositive masking agent 36P and may heat the build material particles 22in contact therewith, and may not be substantially absorbed (e.g., 25%or less of the applied radiation is absorbed) by the non-patterned buildmaterial particles 22.

Additional layer(s) may be formed on the 3D object layer 40 to createthe 3D object. To form the next 3D object layer, an additional buildmaterial layer may be formed as described above. The positive maskingagent 36P is then selectively applied on at least a portion of theadditional build material layer, according to the 3D object model. Then,the entire additional build material layer is exposed to radiated energyin the manner described herein. The formation of an additional buildmaterial layer, the selective application of the positive masking agent36P, and the radiated energy exposure may be repeated a predeterminednumber of cycles to form the final 3D object in accordance with the 3Dobject model.

Negative Masking Agents

In another example of the method 200, a negative masking agent 36N isused instead of the positive masking agent 36P. The negative maskingagent 36N may be selectively applied on at least a portion 34 of a buildmaterial layer 10. In these examples, the negative masking agent 36N mayreflect radiation to which the build material layer 10 is exposed andprevent the build material particles 22 in the portion(s) 34 fromsintering or fusing, while the build material particles 22 that are notin contact with the negative masking agent 36N sinter or fuse to formthe layer 40′.

In some examples of the method 200, the negative masking agent 36Nconsists of a liquid vehicle and an energy reflector. In other examples,the negative masking agent 36N may include other components.

Energy Reflector

The energy reflector is capable of reflecting radiated energy so thatthe build material particles 22 that are in contact with the energyreflector do not sinter or fuse, while the radiated energy sufficientlyraises the temperature of the build material particles 22 that are notunderneath the energy reflector so that those build material particles22 sinter or fuse to become a layer of a 3D object 40′. The energyreflector used may depend, at least in part, on the build materialparticles 22 used.

In some examples, the energy reflector may be a white material, such astitanium dioxide particles (TIO₂) or alumina (Al₂O₃). In other examplesthe energy reflector may be a metallic material, such as silver.

It is to be understood that the examples of the energy reflectorprovided are some examples, and that other energy reflectors may beused.

The amount of the energy reflector that is present in the negativemasking agent 36N ranges from greater than 0 wt % active to about 40 wt% active based on the total weight of the negative masking agent. Inother examples, the amount of the energy reflector in the negativemasking agent ranges from about 0.3 wt % active to 30 wt % active, fromabout 1 wt % active to about 20 wt % active, from about 1.0 wt % activeup to about 10.0 wt % active, or from greater than 4.0 wt % active up toabout 15.0 wt % active. It is believed that these energy reflectorloadings provide a balance between the negative masking agent 36N havingjetting reliability and heat and/or radiation reflection efficiency.

NMA Vehicles

In addition to the energy reflector, the negative masking agent 36N mayalso include a liquid vehicle (i.e., a NMA vehicle) in which the energyreflector is dispersed or dissolved to form the negative masking agent36N.

In some examples, the NMA vehicle may make up about 60 wt % to about 99wt % of the negative masking agent 36N. In other examples, the NMAvehicle may be included in the negative masking agent 36N in an amountranging from about 60 wt % to about 95 wt %, from about 60 wt % to about90 wt %, from about 60 wt % to about 85 wt %, from about 60 wt % toabout 80 wt %, from about 75 wt % to about 90 wt %, or from about 70 wt% to about 80 wt %, based on a total weight of the negative maskingagent 36N.

The solvent of the negative masking agent 36N may be water or anon-aqueous solvent (e.g., ethanol, acetone, n-methyl pyrrolidone,aliphatic hydrocarbons, etc.). In some examples, the negative maskingagent consists of the energy reflector and the solvent (without othercomponents). In these examples, the solvent makes up the balance of thenegative masking agent 36N. In other examples, the NMA vehicle mayinclude other components. Examples of other suitable negative maskingagent 36N components include dispersant(s), co-solvent(s), humectant(s),surfactant(s), antimicrobial agent(s), anti-kogation agent(s), chelatingagent(s), viscosity modifier(s), pH adjuster(s), and the like.

In some examples, the NMA vehicle of the negative masking agent 36N maybe similar to the PMA vehicle of the positive masking agent 36P and/orthe liquid vehicle of the binder agent 28. As such, the NMA vehicle ofthe negative masking agent 36N may include any of the componentsdescribed above in reference to the PMA vehicle of the positive maskingagent 36P or the liquid vehicle of the binder agent 28 in any of theamount described above (with the amount(s) being based on the totalweight of the negative masking agent 36N rather than the total weight ofthe positive masking agent 36P or the binder agent 28).

Printing Methods

In examples of the method 200, a build material layer 10 may be formedas described herein in reference to FIG. 5 .

After the formation of the build material layer 10, and prior to furtherprocessing, the build material layer 10 may be exposed to pre-heating asdescribed above.

After the build material layer 10 is formed, and in some instances ispre-heated, the negative masking agent 36N is selectively applied on atleast a portion 34 of the build material layer 10.

The volume of the negative masking agent 36N that is applied per unit ofthe build material particles 22 in the patterned portion 34 may besufficient to reflect enough radiated energy so that the build materialparticles 22 in the patterned portion 34 do not sinter or fuse. Thevolume of the negative masking agent 36N that is applied per unit of thebuild material particles 22 may depend, at least in part, on the energyreflector used, the energy reflector loading in the negative maskingagent 36N, and the build material particles 22 used.

The negative masking agent 36N may be dispensed from an applicator 32.The applicator 32 may be a thermal inkjet printhead, a piezoelectricprinthead, a continuous inkjet printhead, etc., and the selectiveapplication of the negative masking agent may be accomplished by thermalinkjet printing, piezo electric inkjet printing, continuous inkjetprinting, etc.

After the negative masking agent 36N is selectively applied in thedesired portion(s) 34 of the build material layer 10, the entire buildmaterial layer is exposed to radiated energy.

In some examples of the method 200, the exposing of the build materiallayer 10 to radiated energy is accomplished using a flood energy sourceselected from the group consisting of an infrared lamp, an array ofinfrared emitters, a pulse gas discharge lamp, an array of fiber lasers,a semiconductor laser, a gas laser, an array of the semiconductorlasers, or an array of the gas lasers. One specific example of the pulsegas discharge lamp is a xenon pulse lamp.

The length of time the radiated energy is applied for, or energyexposure time, may be dependent, for example, on one or more of:characteristics of the flood energy source; characteristics of the buildmaterial particles; and/or characteristics of the negative masking agent36N.

The negative masking agent 36N reflects the radiated energy and preventsthe build material particles 22 from absorbing enough of the radiatedenergy to sinter or fuse. The non-patterned build material particles(e.g., in portion 38) absorb the radiated energy and convert theabsorbed radiated energy to thermal energy. In an example, thenon-patterned build material particles 22 absorb enough radiated energyto sufficiently elevate the temperature of the non-patterned buildmaterial particles 22 so that they sinter or fuse. The application ofthe radiated energy forms a layer 40′ of the 3D object.

In some examples, the radiated energy has a wavelength ranging from 800nm to 4000 nm, or from 800 nm to 1400 nm, or from 800 nm to 1200 nm.Radiated energy having wavelengths within the provided ranges may bereflected by the negative masking agent 36N.

Additional layer(s) may be formed on the 3D object layer 40′ to createthe 3D object. To form the next 3D object layer, an additional buildmaterial layer may be formed as described above. The negative maskingagent 36N is then selectively applied on at least a portion of theadditional build material layer, according to the 3D object model. Then,the entire additional build material layer is exposed to radiated energyin the manner described herein. The formation of an additional buildmaterial layer, the selective application of the negative masking agent36N, and the radiated energy exposure may be repeated a predeterminednumber of cycles to form the final 3D object in accordance with the 3Dobject model.

Printing using SLS/SLM

Referring now to FIG. 8 , in examples of the method 300 disclosedherein, after the build material layer 10 is formed, at least a portionof the build material layer 10 is exposed to a laser. In these examples,the layers of the 3D object are formed via selective laser sintering(SLS) or selective laser melting (SLM).

Examples of the method 300 may be used when the build material particlesinclude polymer particles, ceramic particles, metal particles, or acombination thereof.

In these examples, no binder agent 28 or masking agent 36P, 36N isapplied on the build material particles 22. Rather, an energy beam 42 isused to selectively apply radiation to the portions of the buildmaterial layer that are to sinter or fuse to become part of the 3Dobject.

In this example, the source of electromagnetic radiation may be a laseror other tightly focused energy source that may selectively applyradiation to the build material layer. The laser may emit light throughoptical amplification based on the stimulated emission of radiation. Thelaser may emit light coherently (i.e., constant phase difference andfrequency), which allows the radiation to be emitted in the form of alaser beam that stays narrow over large distances and focuses on a smallarea. In some examples, the laser or other tightly focused energy sourcemay be a pulse laser (i.e., the optical power appears in pluses). Usinga pulse laser allows energy to build between pluses, which enable thebeam to have more energy. A single laser or multiple lasers may be used.

In any of the methods 100, 200, 300 disclosed herein, differently shapedobjects may be printed in different orientations within the printingsystem. As such, while the object may be printed from the bottom of theobject to the top of the object, it may alternatively be printedstarting with the top of the object to the bottom of the object, or froma side of the object to another side of the object, or at any otherorientation that is suitable or desired for the particular geometry ofthe part being formed.

To further illustrate the present disclosure, examples are given herein.It is to be understood that these examples are provided for illustrativepurposes and are not to be construed as limiting the scope of thepresent disclosure.

EXAMPLES Example 1

Several examples of the dispersion disclosed herein were prepared. Eachexample dispersion included stainless steel particles (having a D50particle size less than 25 μm) as the build material particles. Up to 50vol % loadings were used for the build material particles. Each exampledispersion also included either isopropanol as the liquid agent or a50:50 mixture by volume of isopropanol and water as the liquid agent.

Each of the example dispersions was successfully sprayed using ahandheld pump spray bottle. Some of the example dispersions werefrequently shaken prior to successful spraying. The example liquidagents quickly (in about 1.0 second or less) evaporated from the layersto form example build material layers.

One of the example build material layers was formed with a dispersionthat included isopropanol and 50 vol % stainless steel particles. Thisexample build material layer was about 30 μm thick and was deposited ona glass substrate. The glass substrate with the example build materiallayer thereon was rotated and shaken. The rotating and shaking had noeffect on the build material particles, which remained well adhered tothe glass substrate. The layer was also scratched to test adhesion, andsome scratches did result. Overall, the results indicated that themolecular and electrostatic attractive interactions between sprayedparticles were strong enough to support a large scale depositionprocess.

Example 2

Two example partially sintered (fused) layers were manufactured usingone of the example dispersions (including isopropanol and 50 vol %stainless steel particles from Example 1). Both of the example partiallysintered layers were fabricated on respective glass substrates with asingle spraying pass of the handheld pump spray bottle and a singleirradiation pulse from a xenon pulse lamp. The single irradiation pulsewas performed to achieve partial sintering so that the layers could beremoved from the glass substrates, handled, and analyzed with scanningelectron microscopy (SEM). The irradiation pulse used to form the firstpartially sintered layer had a dose (i.e., total energy per area (e.g.,Joules per square centimeter (J/cm²)) of the radiated energy) of about22 J/cm², and the irradiation pulse used to form the second partiallysintered layer had a dose of about 24.4 J/cm².

The first partially sintered layer is shown in FIGS. 9A and 9B. In FIG.9A, the first partially sintered layer had been lifted from the glasssubstrate. FIG. 9B depicts the SEM image (with a 20 μm scale bar) of across-section of the first partially sintered layer. FIG. 9B illustratesthat the layer was relatively uniform in thickness and that theparticles had started to sinter together.

The second partially sintered layer is shown in FIGS. 10A and 10B. InFIG. 10A, the second partially sintered layer had been lifted from theglass substrate FIG. 10B, depicts the SEM image (with a 20 μm scale bar)of a cross-section of the second partially sintered layer. FIG. 10Billustrates that the layer was relatively uniform in thickness and thatthe particles had started to sinter together.

Four additional example partially sintered layers were manufacturedusing the example dispersion. Each of the additional example partiallysintered layers was fabricated on a respective glass substrate with asingle spraying pass of the handheld pump spray bottle and a singleirradiation pulse (with an energy exposure (dose) of about 19.7 J/cm²)from the xenon pulse lamp. The four additional example partiallysintered layers had thicknesses of about 20.8 μm, about 24.7 μm, about26.6 μm, and about 24.9 μm (respectively). These results indicateexcellent spraying reproducibility.

Several more partially sintered layers were manufactured using theexample dispersion. Each of the additional example partially sinteredlayers was fabricated on a glass substrate with 1, 2, 3, or 4 sprayingpass(es) of the handheld pump spray bottle and an irradiation pulse at adose of about 19.7 J/cm², about 22.0 J/cm², about 24.4 J/cm², or about26.9 J/cm², from the xenon pulse lamp.

The thicknesses, in μm, of the additional example partially sinteredlayers are shown in Table 2. The additional example partially sinteredlayers are identified in Table 2 by the number of spraying pass(es) andthe dose of the irradiation pulse used to create the partially sinteredlayers.

TABLE 2 1 spraying 2 spraying 3 spraying 4 spraying pass passes passespasses Dose of ~21 μm — — — ~19.7 J/cm² Dose of ~26 μm ~32 μm ~38 μm ~37μm ~22.0 J/cm² Dose of ~25 μm ~34 μm ~36 μm ~35 μm ~24.4 J/cm² Dose of —~37 μm ~33 μm ~29 μm ~26.9 J/cm²

As shown in Table 2, multiple spraying passes increased the thickness ofthe partially sintered layers. However, due to the inability of theradiated energy (from the xenon pulse lamp) to penetrate past athickness of about 38 μm, the thickness of the partially sintered layersproduced using multiple spraying passes ranged from about 30 μm to about38 μm. It is to be understood, however, that full 3D printed parts maybe formed using any of the example 3D printing methods disclosed herein.

Example 3

Four example partially sintered 3D objects were manufactured using theexample dispersion (including isopropanol and 50 vol % stainless steelparticles from Example 1 and Example 2). Each example partially sintered3D object was fabricated on a glass substrate by spraying a layer in asingle pass, exposing the layer to an irradiation pulse, and repeatingthe spraying and the exposing. Each example 3D object had 4 partiallysintered sintered layers.

The thickness of the example 3D objects increased with each layer added.Further, the average layer thickness was reproducible. The four examplepartially sintered 3D objects had total thicknesses (including all 4layers) of about 112 μm, about 111 μm, about 109 μm, and about 104 μm(respectively).

One of the example partially sintered 3D objects formed is shown in FIG.11 . In FIG. 11 , a cross-section of the example partially sintered 3Dobject is shown in a SEM image, with a 50 μm scale bar.

It is to be understood that the ranges provided herein include thestated range and any value or sub-range within the stated range, as ifsuch values or sub-ranges were explicitly recited. For example, fromabout 0.1 mPa·sec to about 50 mPa·sec should be interpreted to includenot only the explicitly recited limits of from about 0.1 mPa·sec toabout 50 mPa·sec, but also to include individual values, such as about0.5 mPa·sec, about 9.75 mPa·sec, about 24.67 mPa·sec, about 47.0mPa·sec, etc., and sub-ranges, such as from about 6.53 mPa·sec to about36.5 mPa·sec, from about 10.25 mPa·sec to about 42.2 mPa·sec, from about18.75 mPa·sec to about 47.79 mPa·sec, etc. Furthermore, when “about” isutilized to describe a value, this is meant to encompass minorvariations (up to +/−10%) from the stated value.

Reference throughout the specification to “one example”, “anotherexample”, “an example”, and so forth, means that a particular element(e.g., feature, structure, and/or characteristic) described inconnection with the example is included in at least one exampledescribed herein, and may or may not be present in other examples. Inaddition, it is to be understood that the described elements for anyexample may be combined in any suitable manner in the various examplesunless the context clearly dictates otherwise.

In describing and claiming the examples disclosed herein, the singularforms “a”, “an”, and “the” include plural referents unless the contextclearly dictates otherwise.

While several examples have been described in detail, it is to beunderstood that the disclosed examples may be modified. Therefore, theforegoing description is to be considered non-limiting.

1. A method for three-dimensional (3D) printing, comprising: sprayingone or more dispersions to form a layer including build materialparticles and a liquid agent; evaporating the liquid agent from thelayer to form a build material layer; and based on a 3D object model,selectively applying a binder agent on at least a portion of the buildmaterial layer.
 2. The method as defined in claim 1 wherein one of theone or more dispersions includes at least some of the liquid agent andat least some of the build material particles, and has a viscosityranging from about 0.1 mPa·sec to about 50 mPa·sec at 20° C.
 3. Themethod as defined in claim 1 wherein spraying involves atomizing the oneor more dispersions by forcing the one or more dispersions through anozzle using a pressure gradient ranging from about 1 atm to about 100atm.
 4. The method as defined in claim 1 wherein one of the one or moredispersions includes at least some of the liquid agent, at least some ofthe build material particles, and a dispersant.
 5. The method as definedin claim 1 wherein one of the one or more dispersions includes at leastsome of the liquid agent and at least some of the build materialparticles, and the at least some of the build material particles arepresent in an amount ranging from about 5 vol % to about 60 vol %, basedon a total volume of the one of the one or more dispersions.
 6. Themethod as defined in claim 1 wherein the build material particlesinclude a mixture of two or more metals or a mixture of a metal and aceramic.
 7. The method as defined in claim 1 wherein the binder agentconsists of a liquid vehicle and a polymer binder. 8.-15. (canceled) 16.The method as defined in claim 1 wherein the build material particleshave an average particle size ranging from about 0.1 μm to about 100 μm.17. The method as defined in claim 1 wherein prior to the selectivelyapplying of the binder agent, the build material layer has asubstantially uniform thickness ranging from about 1 μm to about 200 μm.18. The method as defined in claim 1, further comprising agitating theone or more dispersions prior to the spraying of the one or moredispersions.
 19. The method as defined in claim 1 wherein the binderagent includes polymer particles having a particle size ranging fromabout 1 nm to about 400 nm.
 20. The method as defined in claim 1 whereinthe selectively applying of the binder agent is accomplished using athermal inkjet printhead or a piezoelectric inkjet printhead.
 21. Themethod as defined in claim 1 wherein the binder agent includes a polymerbinder having a weight average molecular weight ranging from about50,000 Mw to about 250,000 Mw.
 22. The method as defined in claim 7wherein the liquid vehicle includes water, a co-solvent, a humectant, asurfactant, a dispersing agent, an antimicrobial agent, a viscositymodifier, and a pH adjuster.
 23. The method as defined in claim 1wherein the evaporating of the liquid agent from the layer isaccomplished by exposing the layer to heat from a thermal heat source.24. The method as defined in claim 1 wherein the evaporating of theliquid agent from the layer is accomplished by exposing the layer toheat from a radiation source.